Typically, silver-base alloys suitable for forming dental restorations in accordance with American Dental Association specifications nominally contain from about 26% to about 28% tin, about 1% to about 2% zinc, about 2% to about 4% copper, with the balance essentially all silver. Alloys of the foregoing type are provided in a finely-particulated powder form of an average particle size usually less than about 100 microns and are adapted to be triturated by the dentist with about 40% up to about 60% mercury, whereby the ensuing amalgamation reaction effects a hardening of the mixture within a matter of a few minutes during which additional shaping, such as by carving, can be carried out for a period of up to about 15 minutes, and the amalgamation reaction is usually complete within about 24 hours. The general requirements of dental amalgams of the foregoing type include a retention of shape, color and appearance; a biological compatability, a restoration of the tooth to its original function and a long, durable operating life. Extensive investigations and tests conducted on dental restorative materials including dental amalgrams has revealed that creep resistance and compressive strength are particularly important mechanical properties which can be directly correlated to the susceptiblity of such restorations to failure or fracture.
A composition having improved marginal fracture characteristics and other mechanical properties is dislocsed in U.S. Pat. No. 3,305,356, whereby a dental alloy is provided having improved compressive strength and abrasion resistance and decreased flow rate or dynamic creep. In accordance with the teachings of the aforementioned U.S. patent, conventional dental alloys are substantially improved in their mechanical properties by dispersing a controlled quantity of non-amalgamatable alloy particles throughout the conventional amalgam, which comprise a discrete dispersion phase chemically bonded in a substantially continuous matrix composed of the conventional amalgam.
While dental amalgams produced in accordance with the teachings of the aforementioned patent are possessed of improved mechanical properties, including higher compressive strength, thereby better withstanding the mastication forces to which they are subjected, it has been observed that the mechanical properties of such alloys deteriorate with aging of the alloy particles prior to amalgamation, and this deterioration is manifested by a significant reduction in the compressive strength of the resultant amalgam. The loss in compressive strength as a result of the deterioration of the alloy powder prior to amalgamation is at least in part attributable to the presence of voids or cracks in the amalgam along the boundaries between the dispersed phase and the continuous matrix. The presence of such porosity in the amalgam has occasioned premature failure of dental restorations produced therefrom, not only because of the reduced compressive strength, but also because of the poor tensile strength of such restorations, which tend to fracture or break as a result of the imposition of bending stresses thereon during mastication.
The present invention overcomes the problems and disadvantages associated with dental amalgams of the type disclosed in U.S. Pat. No. 3,305,356 by providing a metal powder mixture for use in producing dental amalgams which has improved shelf life and does not deteriorate during storage prior to amalgamation, enabling the attainment of dental restorations of continuous high strength and excellent mechanical properties.